SiO2-supported VPO catalysts with different P/V ratios were prepared, used in selective oxidation of butane, and characterized by X-ray diffraction (XRD), laser Raman spectroscopy (LRS), near-edge X-ray absorption spectroscopy (XANES), and P-31 magic angle spinning NMR. Similar to the SiO2-supported catalysts of lower metal loadings studied previously, the maleic anhydride selectivity increased with increasing P/V ratio, to greater than 50 mol % for P/V = 2. XRD and LRS detected predominantly alpha(1)-VOPO4 or a phase that is structurally very similar for both P/V = 1 and P/V = 2 samples. However, within the detection limit of the XANES experiment, the P/V = 1 catalyst contained mostly V5+ under reaction conditions, whereas the P/V = 2 catalyst contained 100% V4+.